1. Field of the Invention
The present invention relates to a system and method for controlling combustion in stratified charge direct-injection spark-ignition internal-combustion engines.
2. Background Art
Stratified-charge direct-injection spark-ignition (DISI) internal combustion engines have been developed to reduce fuel consumption of gasoline engines, but could have even lower fuel consumption if inefficiencies due to incomplete combustion and/or poor combustion phasing could be reduced. Incomplete combustion is usually due to the inability to repeatedly mix the fuel with an appropriate amount of air in the vicinity of the spark plug resulting in under-mixing or over-mixing of the fuel with the air. Undermixed and overmixed regions increase fuel consumption as well as hydrocarbon and carbon monoxide feedgas emissions. Poor combustion phasing results from timing the spark to occur when the appropriate air/fuel mixture is present at the spark plug, rather than at the optimal time in the cycle.
At least two primary design strategies have emerged to control the fuel injection and spark timing for direct injection stratified-charge (DISC) engines: wall-guided and spray-guided combustion systems.
Wall-guided combustion systems generally use a relatively narrow swirl-type fuel injector in combination with a piston bowl to redirect the fuel spray toward the spark plug. To avoid excessive spray impingement on the piston surface at high back pressures (i.e., high pressure in the combustion chamber), and to allow the fuel enough time to rebound from the piston to the spark location, these systems require relatively early injection timing. The long fuel path of wall-guided systems from a side-mounted injector, down to the piston bowl, and back up to the spark plug provides ample opportunity for over-mixing, resulting in less of the fuel being burned. In addition, during operation in stratified-charge mode, a significant amount of fuel may impinge on the piston surface leading to smoke and hydrocarbon (HC) emissions. To improve combustion stability at light load operation (approximately less than 2 bar BMEP), the intake may need to be throttled to reduce air induction, resulting in increased pumping losses and reduced efficiency.
Spray-guided combustion systems primarily use the fuel spray momentum to form the fuel-air mixture. They typically have a close-coupled injector and spark plug. These systems typically require very close timing of the spark and fuel injection, leaving only a small window near the end of injection for firing of the spark plug, which limits control flexibility and provides little time for the fuel droplets to evaporate. This can lead to burning of the droplets and very fuel-rich mixtures, resulting in smoke emissions.
The present invention overcomes various shortcomings of the previous wall-guided and spray-guided combustion systems and balances combustion completeness and combustion phasing while controlling smoke emissions through proper positioning of the spark gap relative to a vortex-induced stratified charge (VISC). The invention also includes piston crown designs and in-cylinder flow control to further improve the efficiency, improve combustion stability, and reduce emissions for a direct-injection stratified-charge internal-combustion engine.
Embodiments of the present invention include a system and method for controlling combustion in a direct injection internal combustion engine that injects fuel directly into a combustion chamber with a wide-angle, hollow-cone spray. The spray momentum forms a vortex that draws fuel vapor out of the spray toward a spark location outside the cone spray to form a combustible fuel-air mixture before the spray impinges on the piston.
To further enhance the stability of the vortex and reduce fuel impingement on the piston while providing a desired compression ratio, the piston crown includes a combustion bowl defined by a raised rim forming an outer periphery of a spark recess or cavity and an anti-spark recess or cavity. The inner peripheries of the cavities are defined by a protrusion generally centered within the hollow-cone spray. In one embodiment, the protrusion is generally linear and extends across the bowl to separate the cavities. In another embodiment, the protrusion is generally dome-shaped.
The rim or outer periphery of the combustion bowl is raised higher around the spark cavity to increase squish flow and enhance the vortex structure. The radius of the spark cavity is selected to envelop the lower portion of the vortex. The cavities preferably are shaped to provide corresponding fuel impingement surfaces substantially equidistant from the fuel injector. In one embodiment, the anti-spark cavity has a larger radius with a lower rim and greater depth than that of the spark cavity to provide substantially equidistant surfaces relative to a centrally located injector inclined slightly from vertical toward the intake ports.
The invention also includes controlling in-cylinder airflow to create primarily swirl flow to further enhance formation of the fuel cloud and mixing of the fuel and air. In one embodiment, the in-cylinder airflow is controlled by at least partially restricting airflow through at least one intake port, which can be accomplished by closing a swirl control valve (SCV) that blocks one of the intake ports in a system having dual intake ports, for example.
The present invention provides a number of advantages. In general, the present invention provides a better tradeoff between combustion completeness and combustion phasing, while controlling smoke emissions by forming and controlling a suitable vortex from the injected fuel spray to induce charge stratification at an appropriate position relative to the spark gap. The piston crown designs and in-cylinder flow control of the present invention help to stabilize the vortex and reduce penetration so that the fuel-air mixture remains around the spark plug for a longer period of time. This expands the ignition timing window to provide greater control flexibility. Because the fuel spray does not need to be reflected from the piston geometry as in the wall-guided systems, smoke emissions are reduced and the fuel can be injected later in the cycle to provide better combustion phasing and improved thermal efficiency. The later injection timing afforded by the present invention also facilitates more complete burning of the hydrocarbons to improve combustion efficiency. The vortex-induced charge stratification produces a more confined stratified charge, which allows stable operation at low loads without inlet throttling, thereby reducing pumping losses and improving efficiency.